The 1200C already has the largest stock tank at 4.5 gallons vs many others at 3.3 gallons. I don't recall seeing any aftermarket tanks that are significantly larger than the 4.5.
Carry a flask or two of gas in your jacketor a portable tank on a luggage rack. Sorry, all kidding aside never found any tanks larger for the sporty, just different shapes as mentioned. About 200 miles on a tank is a fair amt. between fills for the larger stock tank compared to the old 2.5-3.3 gal. tanks.